In the field of data communications, one highly demanding application is the transmission of full motion video, such as may be required for video-on-demand applications. Various protocols which involve compression of digitized video information have been proposed. One such protocol is the MPEG-2 transport stream, as defined in standard ISO/IEC 13818-1: 1996 promulgated by the ISO/IEC. The MPEG-2 transport stream involves transmission of video and audio information in transport stream packets of 188 bytes in length. Each packet includes a header, which contains control information, and a payload, which contains video or audio information. The MPEG-2 transport stream may carry multiple different programs simultaneously. Each packet in the transport stream is associated with a program by a packet identifier (PID) contained in the header. The header is of variable length, depending on whether it contains an adaptation field. The adaptation field contains control information that is not necessarily present in every transport stream packet.
One of the fields that may be contained in the adaptation field is a program clock reference (PCR). The PCR is representation of a system time clock that was used at the source to encode the data in the transport stream packet. The system time clock typically has a frequency of 27 MHz. Each PCR field contains a PCR sample value which represents a count of the system time clock at the time when the transport stream packet was encoded for transmission. PCR sample values for a particular program are received with different transport stream packets having the same PID. A series of PCR samples can be used to reconstruct the system time clock at the decoder end of the communication channel.
The MPEG-2 transport stream may, for example, be transmitted through asynchronous transfer mode (ATM) networks developed for high speed, packetized digital transmission of data, audio and video. One of the parameters that is used in characterizing an MPEG-2 transport stream is PCR jitter. In an ideal communication channel, all transport stream packets, and therefore all PCR samples, are received a fixed time after transmission. However, in a real communication channel, variable delays may be introduced by different channel elements. For example, different transport stream packets may follow different network paths in reaching the final destination. Variations in arrival times produce PCR jitter. PCR jitter is described in standard ISO/IEC 13818-9: 1996.